<?php

$array = array(
    array(
        'id' => 1,
        'cancer_code' => 208
    ),
    array(
        'id' => 1,
        'cancer_code' => 201
    ),
);
$cancer_code_array = array(
    array(
        'cancer_code' => 200,
        "cancer_name" => 'a'
    ),
    array(
        'cancer_code' => 201,
        "cancer_name" => 'B'
    )
);
$cancer_code_new_array=array();
foreach ($cancer_code_array as $cancer_code_key => $cancer_code_value) {
    $cancer_code_new_key = $cancer_code_value['cancer_code'];
    $cancer_code_new_value= $cancer_code_value['cancer_name'];
    $cancer_code_new_array[ $cancer_code_new_key]= $cancer_code_new_value;
}
foreach ($array as $key => $value) {
    $cancer_code=$value['cancer_code'];
    if(isset($cancer_code_new_array[ $cancer_code])){
        $array[$key]['cancer_name']=$cancer_code_new_array[ $cancer_code];
    }else{
        $array[$key]['cancer_name']='';
    }
}
echo "<pre>";
var_dump($array);
echo "<pre>";
die();
?>